What is AJAX

AJAX is an abbreviation for Asynchronous JavaScript and XML. It’s not a technology, rather an approach applied to perform the asynchronous data exchange between the browser and the server.

In this article we are taking a closer look at the main AJAX principles and areas of its application, along with its pros and cons. Synchronous and Asynchronous Requests

Let’s cast light on the types of requests from the browser to the server. They can be either synchronous or asynchronous.

While using synchronous requests:

1. The user performs an action on the page, e.g., likes the publication;
2. The browser transfer the request to the server;
3. The server processes it and transmits the reply to the browser;
4. The reply will look like a fully updated page. The browser reloads the page according to the new data for the user to see the outcomes.

Such an approach to data exchange is called synchronous since the server transmits the response in sync with the new page load. This way, the reloading happens every time a new query appears, however insignificant it is. It can simply be a tick in the appropriate place.

The disadvantages of this approach include:

• The increased time of interaction with the page, i.e., the user needs to wait for the page to reload;
• Heavy load for a server means that it can support less one-time queries;
• Decreased server responsiveness down to a large amount of the transmitted data.

Asynchronous requests allow the server to exchange the data with the browser in background:

1. Each action on the page transmits a point query to the server;
2. The server processes it and gives a response as a fragment of the updated data;
3. The fragment is transferred to the page without the reload. For instance, new posts are added to the newsfeed.

Such an approach is called asynchronous since new information is not uploaded in the form of a new page, yet it’s added upon request. As a result, the user can perform other actions on the page while the data is transmitted between the server and the browser. This way, the work is not interrupted with a page reload.

Asynchronous data exchange is realized through AJAX. With document object model and JavaScript, it performs a dynamic call to server and updates certain objects without reloading the page.

The advantages of this approach are as follows:

• It speeds up the work. The result of the action is seen at once, there’s no need waiting for the page reload;
• The Internet traffic is decreased gratefully to the processing of specific requests;
• The server load is scaled down, so it could process more queries.

When AJAX is Used

AJAX is beneficial when it comes to web applications which need to be interactive and responsive. Let’s consider several case studies.
Form validation and submission

The users apply web forms on a daily basis, i.e., they fill in questionnaires, registration and feedback forms, and sign up for a new service. Gratefully to AJAX, a website can check if the nickname hasn’t been taken yet, and the phone number contains enough digits. Having submitted the form, the user can stay on the same page and keep on reading an article, for example.

If a synchronous has been applied to solve the same issue, the user will receive the message that the nickname is already taken, after having submitted the form. This way, they will have to fill in the form one more time. In case the data is correct, the user will get redirected to the confirmation page. Later on, the site visitor will have to perform extra actions to come back to the previous page. Chats and email services

AJAX helps display conversations in real time while communicating via chats on websites or social media. While the user is writing a message, it gets transmitted to the server immediately. AJAX displays the chat interface on the interlocutor’s page.

Email online services, such as Google, Outlook, etc. apply asynchronous data exchange so that incoming messages could appear in your inbox at once. AJAX also comes in handy when we need to save the draft of the letter and collapse the window with an unfinished message to check out the message history. News feed and online storefronts

Social media, marketplaces and online stores apply AJAX to load the news feed and online storefronts. While the user is scrolling the newsfeed, looking through the goods and reaching the end of the page, the application sends a request to the server so that fresh news and new goods can be seen.

If a synchronous data exchange is applied, the user is forced to get to the next page manually and wait for the information to upload. In case the information is heavy, e.g. it’s visual content, the work slows down a lot. Online pools and ratings

Some platforms and social networks apply AJAX to display votes and ratings in real time. This way, users can leave a rating to postings and vote, while scrolling down the page, without reloading the page. The result is displayed instantly. The vote ranking and the number of ratings are displayed based on the most recent user’s action. What Technologies AJAX is Based On

The approach uses several technologies that realize asynchronous interaction between the browser and the server, including

• JavaScript is a programming language that allows sending a request to any separate element on the page to change its content or qualities. In AJAX, JavaScript is applied to create queries, send them to the server, receive and process the reply and reload the page elements.
• XMLHttpRequest. This object is the base of AJAX which is in charge of HTTP and HTTPS queries to the server and adding the server response data to the page without any reload. XMLHttpRequest is available in JS and is supported by the majority of modern browsers. This is the reason why it is applied by many websites to create dynamic applications.
• XML is a layout language that serves for storing and transmitting data. XML is one of the main formats, yet there also are HTML and JSON replies in a text format.
• DOM is an abbreviation from Document Object Model. Such a model contains all the page elements, connections and interactions between them, their qualities and reactions to the users' actions. Gratefully to such a structure, every single object can be addressed by JS. This is the way AJAX functions.

To sum up, what’s AJAX? It’s an approach which is based on JavaScript and provides asynchronous data exchange between the browser and the server through XMLHttpRequest. XMLHttpRequest transmits the request to the server in HTTP or HTTPS formats, gets a response from it in the form of XML, HTML, JSON and adds new data to the page. It also works with separate DOM objects, so new information is uploaded in the background, without the page reload. The Complexities of Dealing with AJAX

Notwithstanding all the advantages of AJAX, still some restrictions are associated with its realization. They are closely connected to the dynamic content and processing it. Let’s take a closer look at its disadvantages. The need for JavaScript in the browser

AJAX calls for JavaScript enabled in the browser since it enables the query realization and response receival. If the user switches off JavaScript in their browser or applies the screen reader, the dynamic content will be unavailable to them. Errors down to Unstable Connection or Server Work

The unstable work of the server may cause errors connected to the query procession. The replies may vanish or be presented in incorrect order. In case the Internet connection is systematically unstable, the browser might not receive the response from the browser or not transmit the query in general. In both cases, the operation logic is ruined and the user needs to reload the page and start from the very beginning. The Complexity of SEO optimization

Search engines do not see the content that is uploaded dynamically via AJAX. Consequently, it is not indexed, and the content does not appear in the search results, since the search engine can’t see it on the website.

Apart from that, search engines do not always understand the link structure. It happens, for instance, if AJAX is applied to upload different sections on the product page. In case one link leads the user to different page versions, depending on the user location, through AJAX, the system can define them as a doubled content.

Each of these cases causes ranking issues that have a negative impact on SEO. Search engines can hide such pages since they seem to be irrelevant. High Demands on the Backend

It is vital to envisage a certain range of situations and user actions to process asynchronous queries correctly. Each of them requires a scenario on the way the server reacts to different requests, which replies it will generate and if it is capable of processing several requests in a sequence. This makes a project far more complicated and calls for well-qualified experts.